1. Field of the Invention
This invention relates to a method for preparing composite alkaline solid polymer electrolyte from polyvinyl alcohol (PVA) polymer, potassium hydroxide and water, which is reinforced with glass-fiber cloth (GF) to increase its mechanical strength, thermal stability and electrochemical stability. This composite PVA-GF polymer film may be applied in first and secondary thin-film alkaline batteries.
2. Description of the Related Art
Prior literature indicates that polyvinyl alcohol (PVA) is a polymer linked by covalent bonds and hydrogen bonds. It is an amorphous polymer material with low-crystallinity, rotational structure and good flexibility that can block the conduction of electron. PVA is hydrophilic due to its hydroxide group, and has good compatibility with water and potassium hydroxide that also have hydroxide group. The internal conduction of metal ion in PVA polymer is brought about by the strong coupling interaction of metal ion and polymer backbone that produces temporary coordinate bonding, and subsequently the migration of polymer chain. PVA is a polymer material with diverse applications. It is also low-priced and free of any environmental impact.
Fiber glass cloth (see FIG. 1) has similar compositions of ordinary glass. Both are inorganic oxide with silicon dioxide (SiO2) as main component. The glass material is hard and brittle. If it is subjected to high-temperature melting and drawn into glass yarns, it will become flexible with tensile strength increasing by a dozen folds. When used for reinforcement, this material is usually in superfine fibrous state that offers strength and excellent thermal stability. Therefore regardless of the resulting product, there is no residual stress. The broad applications of glass fiber cloth are unparalleled by ordinary glass.
Glass fiber as reinforcement material possess the following properties:
1) High tensile strength which is twice that of steel wire having the same mass.
2) Dimensional stability: Under maximum stress, its unit dimensions change by 3-4% only.
3) High thermal resistance: It retains 50% of tensile strength under the temperature of 343° C.
4) Superior corrosion resistance: It exhibits excellent corrosion resistance and brittleness property when in contact with the majority of chemicals.
5) Excellent fire proofing: It does not burn (generate heat), nor smolder (generate smoke).
PVA polymer electrolyte has extremely high ionic conductivity after processing, but its mechanical strength is not as good as ordinary separators due to structural toughness. This inventor found in the study that the addition of glass fiber cloth in the preparation of PVA polymer electrolyte greatly improved its mechanical strength up to five times that of ordinary separators (see Table 1 and FIG. 3) without sacrificing its conductivity and with the activation energy for ion reaction greatly lowered (see Table 2). It also solved the contraction problem after long-term storage. Due to the high mechanical strength of glass fiber cloth reinforced PVA polymer film, it is less prone to deformation during processing, charging, discharging or packaging of battery. Under scan electron microscope, no pin hole was found on the surface of PVA-GF film. Thus when used in zinc-air fuel cell, it blocks the entry of zinc ion into the air in the cathode when the anode zinc discharges (see Table 3), thereby preventing the occurrence of short circuit. The inventor also found that the electrolyte dipped in PVA polymer was kept in gel state which helps address the leakage problem of battery brought about electrolyte seeping through separator. Moreover this polymer electrolyte retains high conductivity and electrochemical stability under high temperature.
TABLE 1Comparison of Physical Properties of SeparatorsPropertyDrawThicknessWidthStrengthStressElongationspeedType(mm)(mm)(kg)(kg/cm2)(%)Toughness(mm/min)PP/PE0.17101.057.325.625200separatorPVA-GF0.58105.596.122.227.5200polymerelectrolytePVA0.48100.48.1457182.8200polymerelectrolyte
TABLE 2Comparison of Ionic Conductivity (σ) of PVA and PVA-GF FilmElectrolyteσ (S/cm)PVA-GF Electrolyte,PVA Electrolyte40 μm thickTemp (° C.)(M.W: 70,000-80,000)(M.W.: 70,000-80,000)300.15260.1588400.17990.1599500.18750.1615600.19260.1683 70.0.20610.1763Activation energy (Ea)4.020 2.219 (kJ/mole)
TABLE 3Comparison of Discharge of Zinc-Air Fuel Cell with Different SeparatorsTypeUtilization (%)Composite PVA-GFPP/PE 0615CelluloseDischarge currentpolymer electrolyteseparatorseparator150 mA (at C/10)96.0089.3381.00300 mA (at C/5)90.1677.5082.66Theoretical capacitance150015001500(mAh)